Update the README to show correct number of modes
Update the README to show correct number of modes

As seen [here](https://github.com/kubernetes/autoscaler/blob/master/vertical-pod-autoscaler/pkg/apis/autoscaling.k8s.io/v1/types.go#L126-L143) there are four modes in which VPA operates, updating the readme to reflect that change.
kubernetes/autoscaler,kubernetes/autoscaler,kubernetes/autoscaler,kubernetes/autoscaler